San José, Costa Rica, since 1956

Seasons Change: How the Weather

BETWEEN dry and wet seasons inCosta Rica, a transition period occurswhen rainy and dry days alternate. A typicaltransition forecast for the end of Aprilin the Central Valley might read: “Morningsclear, with few clouds. Afternoonspartly cloudy, with isolated rainfall.Temperatures warm, with a lot of haze.”Meteorology is the science dealingwith the atmosphere, weather and climate.Costa Rica’s weather conditions are thedomain of the National MeteorologicalInstitute, a department of the Ministry ofthe Environment and Energy (MINAE).WERNER Stolz is the head of analysisand prediction management at theinstitute.“Meteorology is a living science,” hesays. “It’s not just theory, and it has animportant application in society.”Stolz, 41, a naturalized citizen of CostaRica, was born in Honduras. He studiedphysics and meteorology at the AutonomousUniversity of Honduras and theUniversity of Costa Rica, and has lived andworked here for 10 years.He and his team of five meteorologistsand three technicians evaluate data fromnumerous sources in Costa Rica andabroad. Approximately 400 weather stationsacross the country and 14 satellitestations deliver information on prevailingweather conditions, such as solar radiation,temperatures, air pressure, humidity, windsand rainfall.Additionally, every morning, a radiosonde rises from the institute’s weatherbureau in Alajuela, northwest of San José.Data are completed with information frominternational weather services, includingthe U.S. National Hurricane Center inMiami, Florida.Computer-based numeric programsfacilitate analysis and evaluation of currentand upcoming weather events. The weatherreports are released to the public via fax,e-mail and the institute’s Web site( institute runs weather bureaus inPuntarenas, the principal Pacific port city,and all four of the country’s internationalairports, issuing aviation weather forecasts.Other users include the NationalEmergency Commission, the media, databanks, tourism, agriculture, shipping andwater and electricity providers.Not only does the institute analyze thecountry’s weather, it also researches its climaticpeculiarities.“There are basically two seasons in the Pacific basin,” Stolz explains. “The rainyseason, May to November, and the dry season,December to April. On the Caribbeanside of the country, March and Septemberare the driest months.”Year-round, the weather in Costa Ricais determined by the country’s geographicposition in the Neotropics – 10 degreesnorth of the equator – and by the orientationof the Central Mountain Range, runningnorthwest to southeast. This mountainousbarrier splits the country into twomajor climatic bodies.Temperatures fluctuate relatively littlein the tropics; no more than a 5-degree-Celsius difference exists between the meantemperatures of the warmest and coldestmonths. In general, temperatures vary withelevation rather than time of the year.Stolz says precipitation is the mostimportant weather element. The southernPacific, the northern Caribbean coast andparts of the Central Mountain Range areamong the wettest areas, receiving morethan 4,500 millimeters (180 inches) ofrainfall per year, while the northwesternlowlands are the driest areas in the country.THE principal disturbances that causemost of the weather changes in CentralAmerica, as well as average conditions inthe tropics, are the trade winds, the polarfronts, hurricanes and the so-calledIntertropical Convergence Zone (ITZC).At the equator, two major air masses,one from the north and one from the south,together with ocean currents, form theITCZ, the major climatic heat engine onthe planet, extending five latitudes to thenorth and south of the equator. Intensesolar heating in this region forces air to risethrough convection, resulting in a plethoraof precipitation.Ancient mariners called the ITCZ “thedoldrums,” due to the lack of horizontal airmovement. The zone follows the pathwhere the sun is directly overhead, with alag of approximately two months, and itsannual migration has an enormous influenceon weather patterns in CentralAmerica.“The ITCZ is where the northeasterlytrade winds converge with Pacific winds,”Stolz explains. “It accounts for 60% of theprecipitation in Costa Rica, and its adventheralds the start of the rainy season.”IN late April, when winds blow fromthe southwest, the rains hit the Osa Peninsula,on the southern Pacific coast, first,arriving in Guanacaste by the end of May.During the rainy season, the precipitationlets up for a window in the middle ofthe year. This period is called veranillo,which means “little summer.” Verano, or“summer,” in Costa Rica refers to the dryseason, December to April.In September and October, while theITCZ is stationary above Central America,Pacific showers and thunderstorms bringthe peak in annual rainfall to the country’swestern side. The interaction between theITCZ and hurricanes can generate rainfallof differing durations and strengths.Suction from hurricanes in the Atlanticbasin intensify precipitation by liftingmoisture-laden air over the CentralMountains, dumping even more rain on thePacific side. These disturbances are calledtemporales (rainstorms), referring to anyspate of nearly continuous rain occurringduring the morning and sometimes lastinginto the afternoon.In December and January, when thecold air of the polar region penetrateslower latitudes, it brings several days orweeks of rain, cooler weather and strongwinds, known as nortes (northers), causingthe lowest temperatures of the year. Whilethe Pacific slope enjoys its sunniest, driestperiod, the Caribbean side of Costa Ricacan be bathed in rain.“Very strong winds were among thedisturbances responsible for the devastatingAtlantic rainstorm that occurred inJanuary – the strongest in history,” Stolzadds. “The Limón province experienced344 millimeters (13.5 inches) of rain inonly 19 hours; the average amount of precipitationfor the entire month is 303 millimeters(12 inches).”STOLZ’S prediction for the comingrainy season includes another phenomenonthat might impair weather patterns inCentral and South America.“In Costa Rica,” the weathercastersays, “the first months of the 2005 rainyseason will be influenced by El Niño.Should this climatic event extend into thesecond half of the year, we expect highertemperatures and deficiency of rainfall.”

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